The present invention relates to an electrochemical device for measuring the concentration of oxygen in a liquid alkali metal and, more particularly, to such a device which has a longer life and is more reliable and accurate than prior devices of this nature especially designed to measure the concentration of oxygen in molten sodium.
It is desirable for various purposes to be able to measure the oxygen content of molten alkali metals used in industrial processes and commercial equipment. For example, it is quite important to be able to detect the presence of oxygen in the liquid sodium heat transfer loops of liquid metal fast breeder reactors. The presence of oxygen in the liquid sodium coolant of the primary coolant loop in such a reactor, i.e., the loop which passes through the reactor core, has to be minimized to prevent corrosion and consequent mass transport from the reactor core of radioactive corrosion products. A reliable oxygen monitoring device is also needed for the secondary loop of such a reactor liquid sodium coolant system in order to provide prompt and quantitative detection of steam or water leaks into the sodium.
Oxygen monitoring devices which rely on galvanic principles and ionic conduction have been designed to measure oxygen concentrations in molten metals. Basically, such devices provide an indication of the oxygen content by measuring the electromotive force generated between a reference electrode and a molten metal by the conduction of oxygen ions therebetween through a solid electrolyte. The devices described in U.S. Pat. Nos. 3,776,831; 3,864,231; and 3,864,232 are representative of such devices. Presently available electrochemical oxygen monitoring devices, however, suffer from several deficiencies which make them less than optimum for use in measuring the oxygen content in liquid alkali metals, especially if the alkali metal is, for example, liquid sodium being used as a fission reactor coolant.
One of the primary problems with most presently available devices is that they are not as accurate as desired. That is, most of such devices use air or some other gas as a reference electrode, and in order to provide a sufficiently fast response time the device must be operated at a relatively high temperature, e.g., 800.degree. C. The difficulty with operation of such a device with a gas reference electrode at such a high temperature is that electronic conduction through the electrolyte becomes sufficiently high to interfere with the accurate measurement of ionic conduction through the solid electrolyte. Moreover, high temperature operation substantially increases corrosive action of the alkali metal on the solid electrolyte, thereby reducing the effective life of the device. While it may appear that such problems could be circumvented by operating at a lower temperature, for example, at temperatures around 550.degree. C., such devices generally become irreversible with consequent potential drift during operation.
Also most presently available devices will not provide accurate readings when initially immersed in a liquid alkali metal having a concentration of oxygen in the range of parts per million. The electrolyte material used in such devices is generally comprised of a stoichiometric ceramic composition that has oxygen atoms removed when initially immersed in the alkali metal until an oxygen-depleted composition in equilibrium with the alkali metal is achieved. This removal of oxygen atoms from the electrolyte interferes with the accuracy of the operation of the device until an electrolyte with an oxygen depleted composition is achieved. Generally the kinetics of such removal of oxygen atoms from the electrolyte of the immersed device is very slow and a period of two or three months is required until the electrolyte material is in equilibrium with the alkali metal.